This invention relates to a preparation of diaryl carbonates by direct oxidative carbonylation. More particularly, it relates to the methods for the production of diaryl carbonates by continuous processes.
Diaryl carbonates, and diphenyl carbonate in particular, are valuable monomer precursors for the preparation of polycarbonates by melt transesterification. An advantageous route for the synthesis of diaryl carbonates is the direct carbonylation of aromatic compounds by carbon monoxide and oxygen in the presence of a catalyst.
A wide range of catalysts may be used. For example, U.S. Pat. No. 4,187,242 to Chalk discloses catalysts derived from Group VIIIB metals, i.e., metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or complexes thereof. U.S. Pat. No. 5,231,210 to Joyce, U.S. Pat. No. 5,284,964 to Pressman et al., U.S. Pat. No. 5,760,272 to Pressman and U.S. Pat. No. 5,399,734 to King, Jr., et al. further disclose the use of co-catalysts, including metal co-catalyst species such as cobalt pentadentate complexes and other cobalt(II) complexes with pyridines, bipyridines, terpyridines, quinolines; aliphatic polyamines, crown ethers, aromatic or aliphatic amine ethers, and Schiff bases. These metal co-catalysts are often used in combination with organic co-catalysts such as terpyridines and halide sources such as quaternary ammonium or phosphonium halides.
One catalyst system of particular interest is disclosed in U.S. Pat. No. 5,498,789 to Takagi et al. The catalyst system consists of palladium or a palladium compound, at least one lead compound, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound. Use of a lead co-catalyst yields a process wherein the yield of aromatic carbonate per equivalent of palladium (turnover number of palladium) is high, i.e. greater than about 700. A further improved catalyst system comprises use of hexaalkylguanidinium halides, particularly hexaethylguanidinium bromide, as a halide source. Use of hexaalkylguanidinium halides in amounts essentially equivalent to the amounts of quaternary ammonium halides previously employed leads to significant increases in yield of product (i.e., percentage of hydroxyaromatic compound converted to reaction products).
Despite the potential of the foregoing catalyst system in the direct carbonylation of aromatic hydroxy compounds, there remain certain drawbacks and disadvantages from the standpoint of commercialization, most notably that prior disclosures are directed to batch or batch-flow reactions on small scales. "Batch-flow" processes are disclosed, for example in U.S. Pat. No. 5,399,734 to King et al. In a batch-flow process the reactor is charged with aromatic organic hydroxy compound and catalyst while the oxygen and carbon monoxide are introduced and maintained at constant partial pressures and total pressures. Both batch and batch-flow processes are not economic on commercial scales, and a commercial process for the production of diaryl carbonates should accordingly be adapted in to a continuous flow process.
Such adaptation is not always straightforward. A case in point is adaptation of diaryl carbonate production using the above-described palladium/lead oxide/hexaalkylguanidinium halide catalyst system using continuous flow reactor system 10 shown schematically in FIG. 1. A solution of lead oxide (PbO) and Pd(acetylacetonate).sup.2 in phenol from tank 12 is fed via conduit 16 into common conduit 20. Hexaalkylguanidinium halide in phenol is fed from tank 14 via conduit 18 into common conduit 20. Common conduit 20 feeds the mixture of co-catalysts into reactor 22. However, use of this continuous process system resulted in significantly lower yields than those obtained in batch or batch-flow processes. Accordingly, there remains a need in the art for methods and apparatus for the production of diaryl carbonates by direct carbonylation using continuous flow processes.